Up to 100 people protested against an aerial drop of the 1080 possum poison in the Coromandel area on Saturday.
The demonstration in Thames was against the use of the pest control pellets in Kauaeranga Valley.
Protesters say the water supply for Thames and surrounding settlements is drawn from the area and claim the volumes of 1080 to be used, are too high.
They say the last such operation was three years ago, but 1080 is supposed to be effective against possums for 10 years.
The Department of Conservation says consultation with the community has shown support for the plans.